In a uniflow scavenging 2-cycle engine (a 2-stroke engine) which is also used as an engine of a vessel, a scavenge port is provided in an end portion in a stroke direction of a piston of a cylinder, and an exhaust port is provided on the other end portion side in the stroke direction of the piston of the cylinder. When an active gas is sucked into a combustion chamber from the scavenge port in an intake (air supply) stroke, the exhaust gases generated by the combustion operation are exhausted by being pushed out of the exhaust port by the sucked active gas. At this time, a premixed gas is generated by injecting the fuel gas into the sucked active gas, the combustion operation is performed by compressing the generated premixed gas, and the piston reciprocates within the cylinder by the explosive pressure generated by the combustion operation.
In such a uniflow scavenging 2-cycle engine, when the mixing between the fuel gas and the active gas is insufficient, the concentration of the fuel gas locally increases, and there are problems such as pre-ignition and emission of an unburned gas. Therefore, for example, as disclosed in Patent Document 1, a communicating hole is provided in an inner wall of the scavenge port, and a nozzle tube protrudes into the scavenge port from the communicating hole. Moreover, there is disclosed a technique for securing the mixing time of the fuel gas and the active gas within the cylinder, by injecting the fuel gas into the scavenge port from the nozzle tube, and by starting the mixing between the active gas and the fuel gas before the active gas is sucked into the cylinder.